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Tsubaki was born to Eastern-Asian magi parents in their thirties seeking for ways to further their future in the world of magecraft. They were among those who managed to take the actual machinery underlying the Fuyuki Holy Grail War system, and while there, also obtained partial knowledge on Zouken Matou's magecraft. Seeking to adapt it to their own use, they eventually reached the idea of using magically-modified bacteria to better their host, and decided to use their first born daughter as the test subject. Speaking words like “We will make you into an illustrious mage”, her parents' experiments caused her great pain. They felt no love for her as a person, but only as a vessel to continue their family line.

Although their experiments proved to be fruitful, they lost control of some of the bacteria before she was complete. This deprived her of her consciousness, but her parents had long lost any interest in her as a person. She was hospitalized solely for the sake of making sure that an heir could be born from her. While in a coma, Tsubaki entered her dream world, where she initially suffered from great loneliness. After eventually getting used to it, she continued to explore the city, and she eventually stopped keeping track of time after three months. After one year, the False Holy Grail War began to approach.

It was noted, by Orlando Reeve, that Tsubaki's parents had planned to ally with the police chief, thus implying that they worked for the government organization of America.

Being trapped within her dream world, Tsubaki has no idea as to why she is there. She finds both beauty and loneliness in the world's lack of people. She thinks that the buildings and concrete seem like natural objects springing from the surface of the earth, likening it to a grand, harmonious forest. It took her some time to get over the strangeness of the city, especially the lights in buildings turning on without people. She takes comfort in the stars, and the pleasure she gets from being in the empty city dampens her loneliness.

Originally, Tsubaki's parents had wanted to summon Qin Shi Huang, where one of the emperor's Noble Phantasms had been recovered and was going to be used as a catalyst, and participate in the False Holy Grail War themselves. However, as Tsubaki had obtained her Command Spells and summoned her own Servant, False Rider, this chance was later lost as Rider removed the couple’s original personalities, becoming living dolls following Kuruoka's idea of how they should act, and were taken into the dream world to celebrate Tsubaki's birthday.[1]

During the battle between False Archer and False Lancer, Tsubaki becomes afraid of the noise the battle makes, causing Rider to immediately head off to stop it, which it does.
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Tsubaki continues living in the dream world created by Rider, who also transports other people and animals into the world by infecting them with diseases in the real world, remaining mostly oblivious of the events of the War.

Tsubaki's existence as a Master is eventually discovered by Flat Escardos and his Servant, False Berserker, who were following False Rider's presence. Rather than directly confront Tsubaki, they choose to inform the police force, led by Orlando Reeve, another Master in the War. The Dead Apostle, Jester Karture, the Master of False Assassin, also becomes aware of Tsubaki an enters her dream world in his current form as a small boy. Although Tsubaki is initially surprised by him, she chooses to accept him as her friend.[3]

Jester later informs Tsubaki of the battle between True Berserker and True Archer, and their Masters, that is causing much destruction and damage to both the area and the people. This causes Tsubaki to send Rider to "save" the people present there.[4] Multiple parties eventually converge in front of the hospital where Tsubaki rests, some wanting to save her, some to kill her and some with no interest in her.[5]

Tsubaki is the result of decades of her parents' magical research, only to be used as a vessel for the continuance of their family line and future in magecraft. Both carrying magical pedigrees, they acquired some of the mechanics behind the Holy Grail War and part of Zouken Matou's system of entomomancy. They began to adapt it to their own use with the goal of developing a new way to modify the flesh with even smaller bugs. After much trial and error, they came of with pseudo-entomomancy in the form of magically-modified bacteria meant to be instilled within a young magus to amplify their Magic Circuits as they grow.

Through great agony, they provided the desired effect of amplifying her Magic Circuits beyond measure as she grew up. Her body was hardly altered, but out of control bacteria deprived her of consciousness before she could reach completion. Lesions form all through her body, and they are especially focused around her brain. She eventually enters a vegetative state, and they inhibit growth around that part of the brain without damaging other organs. The bacteria stabilizes and keeps from reaching the point of causing necrosis, but will continue to impede her mental functions. Her parents are fine with that outcome so long as they will be able to use her to produce a proper heir after she develops.

The way that the bacteria has modified her has the effect of making her dreams far more realistic than ordinary and her magical energy projects the sights of the real world into them, granting her an empty world to constantly explore. Though impossible to affect the real world from within a dreamscape, it is possible to project the real world into a dream, though its uselessness makes it a seldom researched field. The dream world is modeled after Snowfield, but without any presence of people. The mechanics of the city still function like normal, with traffic lights functioning and the lights in buildings coming on in the night. She has a sense of time, but there is no sense of taste or smell in the dream. She never hungers, but she is able to sleep. Through the dream world, she is able to inadvertently active her powerful circuits to form a contract with Rider, and Rider facilitates her unconscious magecraft to make those he brings into the dream act as she wishes, making her into the dark horse of the war.